The present invention relates generally to disc grinders for lignocellulosic material. More particularly, the present invention relates to refiner plate segments for such an apparatus.
In high consistency mechanical pulp refiners, the wood fibers are worked between two relatively rotating discs on which refiner plates are mounted. The plates usually have radial bars and grooves. The bars provide impacts or pressure pulses which separate and fibrillate the fibers and the grooves enable feeding of the fibers between the refiner discs. Typically, each refiner plate has a radially inner inlet zone which is adapted for receiving wood chips, previously refined fiber, or the like and at least one radially outer refining zone. The inlet zone performs an initial refining operation thereon to reduce the size of the material, feeds the incoming material into the refining zone and distributes the material around the whole circumference of the refining zone. In most conventional refiners, the inlet zone of the refiner plates either feeds well or distributes well, but rarely achieves both goals effectively.
A large volume of steam is produced between the refiner plates as a result of the refining work. The majority of this steam is exhausted from between the refiner plates via the grooves. However, flow restrictions due to a small plate gap and fiber-filled grooves result in a steam pressure peak between the plates, located radially inward from the perimeter. This pressure peak is a major source of the refining thrust load, and can induce control instability at high motor loads. It is thus desirable that the steam generated during refining be discharged from the refining region as quickly as possible, while retaining the pulp within the region as long as possible.
Since the peak pressure zone is located between the inner and outer radial ends of the refiner plates, the steam is exhausted radially outward and inward from the peak pressure zone via the grooves. The back flow of steam toward the inlet zone of the refiner plate can interfere with the feed of material into the refiner. This generally results in an unstable refiner load and a reduction in pulp quality. The back flow of steam can also carry-over fibrous material into the upstream heat recovery unit. This may result in plugging of the heat recovery unit. The back flow steam may also be lost to the system, resulting in energy losses.
Briefly stated, the invention in a preferred form is a pair of relatively rotating, opposed refiner plates for refining lignocellulosic material. The refiner plates each have radially inner and outer edges and an inlet zone extending radially outward from the inner edge. The inlet zone of the first refiner plate, which is rotatable in a direction of rotation, has a radially inner portion and a radially outer portion. The inner portion includes a plurality of curved breaker bars. Each of the breaker bars curves in a direction which is opposite to the direction of rotation from an inner end disposed adjacent the inner edge of the first refiner plate to an outer end disposed adjacent the outer portion.
Each of the breaker bars has a leading edge having a feeding angle xcex1 at any given point therealong. The feeding angle xcex1 is defined by the angle between the leading edge at the given point and a radial line passing through the point. The feeding angle xcex11 at a point adjacent the front end of the breaker bar has a value between 0xc2x0 and 30xc2x0. The feeding angle xcex12 at a point adjacent the outer end has a value between 60xc2x0 and 90xc2x0. Each of the breaker bars has a top surface defining a height substantially equal to one-half of the refining gap formed between the opposed refiner plates.
The inlet zone has an arc length xcexS and each of the breaker bars has an arc length xcexB. The sum of the arc lengths of the breaker bars is at least 50% of the arc length of the inlet zone, preferaby between 60% to 100% of the arc length of the inlet zone, and more preferably between 60% to 80% of the arc length of the inlet zone.
In a first embodiment, the outer portion of the inlet zone of the first refiner plate has a smooth surface. In a second embodiment, the outer portion of the inlet zone of the first refiner plate has a plurality of outwardly extending low profile protrusions.
The inlet zone of the second refiner plate also has radially inner and outer portions which are disposed oppositely to the inner and outer portions of the first refiner plate. The outer portion of the inlet zone of the second refiner plate has a plurality of dams. Each of the dams has an upper ramp surface extending radially outward from an inner end to a head disposed adjacent an outer end. The outer end of each dam has a curved profile. The ramp surface may be either curved or flat. The inner portion of the inlet zone of the second refiner plate may have either a smooth surface or a plurality of outwardly extending, low profile protrusions.
It is an object of the invention to provide a refiner for refining lignocellulosic material having new and improved rotor and stator plates.
It is also an object of the invention to provide new and improved rotor and stator plates in which the stator plate directs back flowing steam and material onto the rotor plate which restricts the back flow of material and pumps the back flow of steam forward.
Other objects and advantages of the invention will become apparent from the drawings and specification.